Many applications exist requiring the precision deposition of various elements or compounds in extremely thin layers onto substrates. The best known of these applications is used in creating microchips and other components for the semiconductor industry. Molecular Beam Epitaxy (MBE) is capable of precision deposition and is generally accomplished by separately heating a substrate and one or more source materials in separate cells. Upon heating the source material in an effusion cell the vapor pressure is raised, forcing the vaporized source material out of the effusion cell and into a vacuum growth chamber where a beam of the vaporized source material or effluent flowing from the effusion cell condenses onto the substrate. Deposition of the source material onto the substrate results in crystals of the source material growing onto the substrate. By varying the temperature of the effusion cell, the flux rate of the molecular beam, and hence the rate of deposition, can be controlled to a limited extent. Using this heat variable MBE technique requires manually adjusting the temperature rate of the effusion cell, which process is somewhat by trial and error. This results in slowed production times, and often an inability to accomplish a graded deposition. Graded deposition is required in certain situations where the underlying substrate is not compatible with the desired outermost source material. An intermediate layer, which is compatible with both substrate and outer source material must be used. However, deposition of an intermediate layer often requires a gradual and continuous change in materials. Thus, many situations exist where the heat variable MBE technique is unworkable.
A solution to the problem of controllably varying beam fluxes involves using a valve system interposed between the exit of the effusion cell and the growth chamber. By opening the valve following raising the vapor pressure in the cell, it is possible to achieve a uniform and precision molecular beam flux into the evacuated growth chamber.
Several valve mechanisms have been proposed to controllably vary beam fluxes. Vapor sources such as effusion cells that modulate the flux with an integral metal needle valve or a movable metal shutter or screen are known in the art of Molecular Beam Epitaxy, as shown in U.S. Pat. No. 5,080,870, which is incorporated herein by reference. The majority of molten metals and resulting metal vapors are particularly reactive with other metallic species at elevated temperatures. Thus, the current sources with metal valves or metal shutters may only be used with a few relatively nonreactive metallic species. If the metallic elemental source material reacts with the valve mechanism or movable shutter, the vapor source fails and the vapors used for the vacuum deposition of thin films become contaminated. Thus, what is needed is an effusion cell capable of modulating the flux containing a corrosion resistant mechanism.